Their discussion is based on the recent review article “Methods and models for metabolic assessment in mice” by Drs Pacini, Omar, and Ahrén, published in J Diabetes Res 2013 (986906)..

Professor Bo Ahrén
Lund University

Bilal Omar, Senior Scientist
Lund University

Their discussion is based on the recent review article “Interaction between cytokines and inflammatory cells in islet dysfunction, insulin resistance and vascular disease” by Drs Imai, Dobrian, Nadler and others, published in Diabetes, Obesity and Metabolism 15 (Suppl. 3): 117-119, 2013.

Bo Ahrén: We have known for many years that insulin secretion is a key component of the function of the pancreatic islets, and impaired insulin secretion (in relation to the requirement) is a key feature of type 2 diabetes. It has therefore been of great importance to develop methods for studying and quantify insulin secretion, and great efforts have been put into such projects by many researchers. However, methods for detecting and quantifying insulin secretion in mice have also been developed.

Why is it of interest to study insulin secretion in mice?

Bilal Omar: As deficient insulin secretion is a key defect in both type 1 and type 2 diabetes, it is important to study both how insulin deficiency occurs and how to improve insulin secretion therapeutically. Mice offer both flexibility and reproducibility, and terminal studies can be done on mice which cannot be done on humans.

Bo Ahrén: What is the best method to measure insulin secretion in mice and how can this be undertaken?

Bilal Omar: A good method is to study the insulin response to glucose, IVGTT or hyperglycemic clamp. Each of these techniques allows for different aspects of insulin secretion to be studied in detail. For example, an IVGTT allows a direct measure of islet response to glucose in vivo, without having to consider the absorption of glucose by the GI tract or the influence of GI hormones on insulin secretion. However, IVGTT has a drawback in that only the first phase of insulin secretion can be evaluated. The hyperglycemic clamp method allows for the measure of both first and second phase insulin secretion in intact mice. However, it also important to be able to measure the insulin response to oral glucose as the normal physiological route of nutrients is orally.

Bo Ahrén: Yes, by using an intravenous glucose administration, it is the direct effect of glucose on the beta cells that is important for the insulin secretory response. After oral ingestion of glucose the situation is much more complex. Glucose levels are, of course, also raised after an oral glucose load due to absorption of glucose, but in addition to this, glucose induces several other responses which also are important for the final insulin secretion. Most well known is the release of the incretin hormones, which are mainly GIP and GLP-1. These hormones are then released into the blood and circulate to reach the islets and stimulate a direct incretin response which adds to the direct effect of glucose. On top of that, however, there is also a neural response after oral ingestion, which is induced by gut motility, nutrients in the gut, and the gut hormones (including the incretins). This neural response also results in a stimulation of insulin secretion. To study this complex and integrated effect, an oral glucose administration (OGTT) is better than an intravenous administration.

Since OGTT reflects the oral administration of nutrients, it is the most physiological test. After administration of glucose, which is usually achieved by glucose instillation by gavage into the stomach, glucose is absorbed in the intestinal tract and enters the splanchnic circulation and then into the systemic circulation. This raises circulating glucose, which together with the gut hormones and autonomic nerves, which also activated by oral glucose, stimulates -cell function.

Glucose and insulin are then analyzed in samples taken after the oral administration of glucose, and insulin secretion can be evaluated by the results of these analyses through various mathematical models, as we elaborate on in our article. One further important mechanism of islet function is, however, to adapt insulin secretion to insulin sensitivity, since under normal conditions insulin secretion is adjusted to the requirement. For example, in obesity there is a development of insulin resistance and the pancreatic islets adapt to this by increasing insulin secretion. This means that by comparing insulin secretion in a lean versus an obese individual (being a person or a mouse) requires some sort of normalization to the requirement to achieve accurate results. How can that be done?

Bilal Omar: The Disposition Index (DI) is just the type of measure to address this. A healthy, insulin-sensitive mouse, or human, should display an incremental increase in insulin secretion in response to glucose for each incremental decrease in insulin sensitivity. In this manner normal glucose tolerance is maintained. This is known as disposition. In mice for example, the insulin secretory respose to the IVGTT can be related to a dynamic measure of insulin sensitivity (Si) in the same individual. The product of the two measures gives the disposition index. If insulin sensitivity declines and the insulin secretion does not increase to compensate there will be a decline in the disposition index value and this could signal increased risk for developing type 2 diabetes.

Bo Ahrén: What are the weaknesses and advantages to use these methods in mice?

Bilal Omar: A weakness is that it is mice and their physiology and anatomy are different to our own especially in terms of their ability to adapt their beta cell mass and insulin secretion to declining insulin sensitivity. Beta cell function declines but does not fail in mice as it does in humans. Another is the technical difficulty. It requires a very skilled technician and much practice to implement these procedures. It also requires special mathematical skills to model and interpret the data. One clear advantage is reproducibility. Inbred strain mice have almost no genetic variability to the biological variation in the experiments. Another related advantage is power. Mice have a relatively low cost for the amount of data that the experiments can generate and they give possibility to use many giving us statistical power to tease out differences and develop new hypotheses.

Bo Ahrén: So, in summary, we have discussed the main tests for metabolic assessment in the mouse including their techniques. In our article we discuss this in more detail and also provide a list of previous applications of these methods.